This invention relates to a process for preparing a foamed article of thermoplastic resin and also to a die for preparing the same.
It is known in the art that a foamed article having a given shape in cross section, such as a sheet, tube and the like, can be prepared by mounting a die on an extruder and extruding through the die a thermoplastic resin containing a foaming agent. Further, it is also known that a foamed article is prepared by using a die having a resin discharge face provided with a number of apertures, extruding a foamable resin through the apertures to form a corresponding number of strands, gathering the strands in a parallel relation while the strands are still in a softened state, and coalescing the strands to form an intergrated article. Japanese Patent Publication No. 35-10518 and U.S. Pat. No. 3,121,130 disclose processes for preparing a uniformly foamed article by distributing the apertures uniformly on the resin discharge face of the die. Furthermore, Japanese Patent Publication Nos. 47-40293, 47-40294 and 47-51945 disclose processes for preparing a foamed article having different density portions, either by distributing the apertures non-uniformly in the forward face of the die, or by allowing the resin to flow non-uniformly through the apertures, or by restricting the spaces wherein foamable resin strands can be foamed to form expanded strands having different densities.
These processes are suited for preparing a product having a small thickness such as a sheet. The processes are also suited for preparing a tube having a small thickness. However, if the processes are used for preparing a product having a circular or square cross section of more than 20 mm in diameter or side length, great difficulty is encountered in that the product contains sporadic holes between the foamed resin strands constituting the product. In other words, when it is intended to obtain a product having a large thickness, the individual strands can be foamed as desired according to the processes, however, it is difficult to fuse or coalesce the strands together, and, as a result, the sporadic holes are produced between the strands.
The inventors tried to eliminate the holes formed between the strands and to obtain a foamed article of a large thickness having no holes, or no hollow portions between the coalesced strands. The inventors believed that the holes were formed by the foaming agent, which was at first contained in the resin strands and then evaporated from the strands when the strands were coalesced together. The inventors also believed that the holes were caused by an insufficient coalescing force when a number of resin strands are coalesced together. In other words, the holes were formed by an insufficient pressing force which is exerted from the outside when the resin strands were coalesced. As a result, the inventors tried to increase the pressing force and to coalesce the resin strands with a larger force, but found that this only lead to a decrease in the degree of expansion of the foamed article and did not result in eliminating the holes. Furthermore, the inventors tried various other experiments, but could not eliminate the holes.